1. Field of the Invention
The present invention relates to a thermistor, which can be, for example, suitably mounted on an exhaust pipe of an internal combustion engine for detecting the temperature of an exhaust gas.
2. Description of Related Art
Known in the prior art is a temperature detector, including a temperature detecting element, which is used in an appliance such as a boiler or an internal combustion engine to detect the temperature of the hot water or exhaust gas. In Japanese Un-Examined Utility Model Publication No. 55-130236, a device, which includes a sensor unit arranged in an exhaust pipe of an internal combustion engine, is provided for detecting the temperature of an exhaust gas or the temperature of a catalytic converter, and a module for detecting a temperature higher than a predetermined value causes an outside device to commence its designated operation. In this prior art, the output terminals of sensor are connected to the above-mentioned module directly. In other words, an independent connector is not used for obtaining an electric connection between the sensor and the module.
Furthermore, in Japanese Un-Examined Patent Publication No. 51-140669, a fixed resistor is connected in series with the output wire from the temperature detector in such a manner that a resistance value compensation function is provided when the temperature to be detected is high.
In short, in the prior art Japanese Un-Examined Utility Model Publication No. 55-130236 and in Japanese Un-Examined Patent Publication No. 51-140669, a function for controlling the output value is provided independent from the part which responds to the temperature to be detected. However, in these prior art, only an outside resistor is proved, which is not able to fully compensate for the variation in the value of the resistance of the resistor (thermistor) for detecting the temperature. Thus, the precision of the temperature measurement is reduced.
Furthermore, in another prior art, Japanese Un-examined Patent Publication No. 5-34208, a thermistor type of temperature detector, of a wide range measurement type, is disclosed. In this prior art, two thermistors having different thermistor-B-constants are connected in parallel so that the combined resistance value of the thermistors can vary over a wide range. In this case, in accordance with the temperature difference between a low temperature and a high temperature, the value of the resistance can be varied over a range of, for example 0.1 K.OMEGA. to 200 K.OMEGA.. In this case, a temperature sensing circuit, including a pull-up resistor, is provided for obtaining a voltage output corresponding to the detected temperature. However, in the prior art, such a pull-up resistor is a fixed resistor type, and, therefore, with respect to a large change in the resistance value of the thermistor due to the change in the temperature, it is difficult to obtain the optimum value of the pull-up resistor to obtain the maximum change in the voltage level with respect to the change in the temperature. Thus, the precision of the temperature detector at a low or a high temperature region is reduced.
Furthermore, a usual thermistor-type sensor is a thermistor element constructed on a chip made of a ceramic plate, with electrodes formed thereon, and covered with a glass plate. This sensor is referred to herein as an A-type sensor. Also known is a B-type thermistor, where a thermistor element is embedded in a sheath, such as a metal pipe, such as those disclosed in a Japanese Examined Patent Publication No. 52-7535 and Japanese Un-Examined Patent Publication No. 50-118294.
Furthermore, in order to increase the speed of the temperature measurement response, the thermistor sensor of the A-type is improved in that a gap for allowing the temperature detecting element to contact the gas is used as disclosed in the Japanese Examined Patent Publication No. 52-7535. Japanese Un-Examined Utility Model Publication No. 5-34543 is improved further in that the metal sheath is flattened in order to increase the response speed. However, an arrangement of the temperature sensor with respect to the flow of the gas may adversely affect the response speed of the temperature measurement.